1. Field of the Invention
The present invention relates to improvements in a heat roll for electrophotography in which a bonding layer, an insulating layer, a resistance layer, and a surface insulating layer are provided sequentially on the outer surface of a core formed into a hollow tubular shape, and heat is generated by energization of the resistance layer so as to fix a toner image on a recording medium.
2. Description of the Prior Art
A conventionally known heat roll used in an electrophotographic printing system for thermally fixing a toner image transferred onto a recording medium such as copying paper is arranged such that a halogen lamp is provided in a hollow tubular core as a heat source to effect heating. With a heat roll of this type, however, there are drawbacks in that the rate of power consumption is large, and that a long warming-up time required until the start of copying after energization. In addition, there is another drawback in that the heat roll outside diameter cannot be made sufficiently small since the lamp is provided inside it. As a means of overcoming this drawback, a direct-heating heat roll is known in which a heating resistor is arranged on the outer surface of the core. FIG. 5 is a partial cross-sectional view illustrating one example thereof, in which an insulating layer 4 and a resistance layer 5 are provided on the outer surface of a core 2 formed into a hollow tubular shape via a bonding layer 3. Electrode rings 6 are respectively fixed to opposite end portions of the resistance layer 5 and are electrically connected to the resistance layer 5. A feeder brush 7 is disposed such as to slidably abut against the outer periphery of each of the electrode rings 6. A surface insulating layer 8 is disposed on the outer periphery of the resistance layer 5 to electrically protect the insulating layer 5 and prevent the insulating layer 5 from becoming damaged by external force. Generally, a ceramic is used for the insulating layer 8 and the resistance layer 5. Incidentally, in cases where the core is formed of an insulating material, the bonding layer 3 and the insulating layer 4 may not be provided.
If a ceramic is used for the insulating layer or the resistance layer, the material of the core, should have a coefficient of thermal expansion which is close to that of the ceramic (generally, 5 to 10.times.10.sup.-6 /.degree. C). If an aluminum alloy which has a large coefficient of thermal expansion is used as the core, cracks occur in the ceramic owing to repetition of thermal loading during production or usage thereof, resulting in deterioration of its electrical properties and breakage of the resistor. In terms of economic efficiency, a ferrous alloy (e.g. mild steel, ferrite-based stainless steel, or martensite-based stainless steel) is most desirable. In addition, there are cases where an insulting ceramic formed of alumina or the like is used.
However, if a ferrous alloy, alumina or the like is used, since its coefficient of thermal conduction is smaller than those of aluminum, steel, and an alloy thereof, the temperature of each portion of the roll does not become uniform, so that there has been drawbacks in that faulty fixing occurs, and that the temperature of certain portions rises unacceptable.